Silaheterocyclen XXXVII.: Die Kombination Alkeninylhalogensilan/Lithium-tert-Butyl als Synthesebaustein zum Aufbau von Silaheterocyclen mit kumulierten CC-Doppelbindungen

A series of differently substituted alkeneinylhalogensilanes R2Si(X)–CC–C(Me)CH2 (X=Cl: R=Me (2), Cl (3), Ph (4); X=F: R=Me (5), Mes (6), Tip (7)) is reacted with t-BuLi in solvents of different polarity to give mainly E/Z-isomeric 2,4-bispentenylidene-1,3-disilacyclobutanes (11, 23 and 31) and E/Z-isomeric 2,4,6-trispentenylidene-1,3,5-trisilacyclohexanes (12 and 24) competitively. Obviously the four- and six-membered silacycles are formed stepwise by intermolecular coupling reactions of the lithiated precursors R2Si(X)C(Li)CC(Me)–t-Bu which preliminary result from the 1,4-addition of the lithium organyle to the alkeneinylhalogensilanes. Alternatively, silacumulene formation (R2SiCCC(Me)CH2–t-Bu) might be discussed, but the isolation of Cl3Si–C(CC(Me)CH2–t-Bu)–CC–C(Me)CH2 (22) as coupling product starting from trichlorosilane 3 and t-BuLi and high molecular multistep coupling products starting from the fluorinated precursors 5 and 7 strongly favour the the coupling reactions over silacumulene formation. Consequently, all attempts to isolate a stable silacumulene from the silicon sterically overcrowded starting compounds 6 and 7 failed.